11d 7170 sp 5 on Server 2003, this is not happen during backup or restore, and I have seen many times that backup exec job engine service is stopped.

details of event viewer are mentioned below:

Backup Exec Alert: Job Cancellation
(Server: "xxxxxx") (Job: "Backup_xxxxxx") The job was canceled because the response to a media request alert was Cancel, or because the alert was configured to automatically respond with Cancel, or because the Backup Exec Job Engine service was stopped.